Conventionally, there is a known method for bonding a wafer made of Si and a wafer made of glass together by applying a voltage using a electrode for the glass wafer as a cathode while both the wafers are contacted with each other and heating to as high as about 400° C. to 500° C. (anodic bonding). In the conventional technique, the wafers are transported in the atmospheric air, so that organic substances unavoidably adhere to surfaces of the wafers. The strength is as weak as 3 MPa at 200° C. as illustrated in FIG. 5, and therefore, the strength is increased to about 9 MPa by heating at a high temperature of about 400° C. In other words, conventional anodic bonding requires high-temperature heating as well.
Patent Document 1 discloses an exemplary room-temperature bonding method of etching metals using an Ar ion beam and bonding the surface-activated metals together at room temperature. In this method, however, organic substances or oxide film is removed from a surface to prepare a surface which is electrically activated with metal dangling bond so that bonding is performed due to an atomic force, and therefore, firm bond cannot be achieved for Si (semiconductor), and particularly glass and SiO2, which are oxide materials.
When objects to be bonded are arranged facing each other and are subjected to a plasma treatment as disclosed in Patent Document 2, a electrode for one of the objects to be bonded inevitably serves as a plasma electrode, so that reaction gas ions are accelerated and strike the object to be bonded on the plasma electrode. Therefore, this technique is suitable for physical etching which removes an organic substance layer, but not for a chemical treatment using OH groups or the like, because of being excessively strong thereto.
As an application in which anodic bonding is often used, a method of sealing from one or both sides is often used for high-frequency devices or MEMS devices as illustrated in FIG. 8.
Patent Document 1: JP S54-124853 A
Patent Document 2: JP 2003-318217 A